I had applied the wallpaper on the interior wall quite awhile ago, and I fully admit to procrastinating on the other two walls. I had never wallpapered around off angles like the one created by the interior chimney extension.
I cut plain paper templates and ended up with one long piece for the wall and one smaller piece for the chimney. I first cut the long piece from the scrapbook paper - Cute Little Diamonds by The Paper Company - and applied it to the wall with Yes paste. I then glued the smaller piece to the chimney.
Now, that's a nice seam, don't you think? I'm rather proud of it! I guess all those years of sewing come in handy for pattern matching.